1. Field of the Invention
The present invention relates to a light beam scanning apparatus preferably applicable to a laser beam printer and the like, and more particularly to a light scanning apparatus without having an f.theta. lens which is generally used for obtaining constant speed for scanning when scanning is performed by deflecting light beam with a deflecting means such as polygon mirror and holoscanner.
2. Brief Description of the Prior Art
FIG. 1 shows a light beam scanning apparatus provided with an f.theta. lens adopted in the conventional laser beam printers. The apparatus comprises a laser diode 41 for irradiating laser beam, a collimator lens 42 for converging the laser beam on the point of image formation, a cylindrical lens 43, a polygon mirror 44 or a holoscanner to be utilized as a deflecting means for a scanning laser beam, an f.theta. lens 45 and a pair of turning mirrors 46,47. When the polygon mirror is rotated in the direction of arrow A, the point of image formation of the laser beam is moved in the direction of arrow B on a photoconductive drum 48 and scanning is performed thereon.
The cylindrical lens 43 is arranged to eliminate the effect of beam deflection caused by inclination of the surface of reflection by making good use of a combined effect with the toric function of the f.theta. lens 45 by horizontally diaphragming the laser beam on the surface of reflection of the polygon mirror 44. The f.theta. lens 45 is capable of performing f.theta. function to convert circular uniform motion of the polygon mirror 44 to a linear motion with constant velocity on the photoconductive drum 48, which will be described referring to FIG. 2.
The laser beam deflected in the .theta. direction by the polygon mirror 44 is formed at the position of image height f..theta., and a degree of equalized pitch is secured by making the very small scanning width .DELTA..sub.0 in the central portion equal to the very small scanning width .DELTA..sub.1 at the edge of scanning portion. The f.theta. lens 45 also functions to correct the curve of the image surface. If there is no f.theta. lens 45, the image forming surface forms a circular arc with a point of deflection on the polygon mirror at its center as shown by a broken line in FIG. 2 thereby creating the amount of defocus .delta. in the edge portion when focus is made in the central portion.
Japanese Published Examined Patent Application No.18163/1986 discloses a light scanning apparatus with an f.theta. lens removed, wherein f.theta. function is electrically corrected. However, curve correction on an image forming surface is not being made. While, Japanese Published Unexamined Patent Application No.4109/1977 discloses an apparatus utilizing a parabolic mirror in place of an f.theta. lens.
In the case of FIG. 1, a polygon mirror 44, an f.theta. lens 45 and the like are disposed in superposed relation on the above of the scanning width, however, Japanese Published Unexamined Patent Application No.172013/1985 discloses an apparatus wherein a laser beam emitting source, a hologram deflection device, an f.theta. lens and the like are disposed on a side of a scanning light path.
However, a laser beam printer utilizing an f.theta. lens is costly because the f.theta. lens is an expensive part comprising a multiplicity of high precision lenses. The apparatus which is arranged to electrically correct the f.theta. function is complicated for controlling and the correction of curve on the image forming surface can not be made without problems on the precision. Further, an apparatus utilizing a parabolic mirror needs a larger space to be disposed therein and the cost can not be reduced substantially.
In a conventional laser beam printer wherein an f.theta. lens is utilized, an optical system is constructed compactly by utilizing turning mirrors 46,47 since the light path length from the polygon mirror 44 to the photoconductive drum 48 is as long as or more than 200 mm. However, for making a laser printer compactly, a problem of space for accommodating the optical system arises as the light path length becomes longer. Under such condition, it is necessary to arrange a cartridge in an electrophotographic section not to interfere with the light path between a laser beam scanning optical system and an electrophotographic section provided thereunder. Further, a moving space required for attaching and detaching the cartridge has also to be taken into consideration which eventually makes it difficult to limit the height of a laser printer.
Japanese Published Unexamined Patent Application No.279869/1986 discloses an apparatus which is arranged to fully open upwardly into an upper unit from a lower unit which stores a laser beam scanning system when a cartridge disposed around a photoconductive drum is exchanged. If such a structure is adapted, no consideration need to be given for space for moving cartridges, however, a large-scale structure for opening and closing the upper unit needs to be provided thereby inviting a high cost in producing the apparatus.